Steam loco operation

[Triplex]

Modeling the transition era, you're narrowing things down to a 10 year time frame, 1950-1960.

Depending on the road, the transition era began a lot earlier than that. Many roads were buying diesel switchers, and some streamliners, in the 1930s. Some roads (GM&O, Monon, Susquehanna, NYO&W, etc.) were completely dieselized by 1950.

Keep steam for nighttime freights and express passenger trains, but use first-generation diesel electric for daytime freights and the odd limited run. This would be typical for many roads extant in 1953.

Express passenger was usually the first to go diesel. Exceptions include N&W and GTW.

ive looked at diesels that were built in the 50's such as a few f 7/9 sd 7/9 and 24 or gp 7/9 as for rolling stock any thing that would fit in the time frame, we will need coal,box,tank and coverd hoppers.

The SD24 is the latest of those, coming in 1959.
The question is, what do you want covered hoppers for? They were common for sand and cement in the transition era. However, grain moved mainly bagged in 40' boxcars. The changeover of that traffic to 3- and 4-bay covered hoppers began in the 60s, and lasted until the 90s in some places.